Immune Tolerance Induction by a Tumor Specific Antigen. Significant advances in the knowledge of the immunology of cancer have occurred. In humans, low responses rates have hampered broader usage of this approach to cancer therapy. One very likely reason for this id the presence of immune tolerance to tumor specific antigens. To be able to design strategies to circumvent immune tolerance, and thereby design more effective immunotherapy, the mechanisms of tolerance in tumor immunity need to be defined. The study proposed here will utilize transgenic mouse technology to develop a model of immune tolerance induction by neoplasms. Specifically, a transgenic mouse model which expresses two transgenes will be used, the gene for SV40 T-antigen in pancreatic acinar cells which results in the development of pancreatic tumors; and the gene for the T cell receptor (TCR) which recognizes SV40 T-antigen, which is expressed on 90% of T cells. Preliminary observations of these mice demonstrated that the relevant T cells are gradually deleted over time, which explains why tumors are not rejected. In the study proposed here the mechanisms responsible for this depletion will be defined. Initial experiments will be performed to determine whether thymic clonal deletion, peripheral clonal deletion, clonal anergy, or TCR modulation occurs. This will be accomplished with adoptive transfer experiments, FACS analyses, and antigen induced proliferation and killing assays. Next, an in vitro tolerance induction assay will be developed using T cells from the TCR transgenic mice, and a tumor cell line derived from tumors arising from SV40 T-antigen transgenic mice. The presence or lack of relevant co-stimulatory molecules, cytokines, or cellular interactions will be defined. Finally, an in vivo model, will be developed to study tolerance, applying manipulations based on the results obtained from the in vitro model.